The proliferation of cellular radiotelephones, their technology and their usage, has raised the need of locating these cellular radiotelephones during a call. The dispatching of police/fire/ambulance services to a location can be accomplished by quickly and accurately determining the location of a cellular radiotelephone engaged in a 911-emergency call. Other applications could include tracing of unauthorized radiotelephone usage, providing location data for location sensitive billing, aiding in police investigations, and the like.
Previously installed cellular systems have little capability in this regard. For example, in Advanced Mobile Phone Service (AMPS) systems, the specific cell in which a radiotelephone is located can be identified by determining which base station antenna was used to serve the call. However, a cell can be as large as 3-5 miles in radius, making this information practically useless for accurately determining the location of the radiotelephone within the cell. Many of the dense urban cell sites are now much smaller and many of the urban/suburban cell sites are now sectored. Sectored antennas are used to limit a channel's service area to just one sector of a cell, thus, the coverage areas of a sectored cell are now smaller. However, the area even in these sectored cells can still be more than one square mile. Since it is often necessary that the location of the cellular radiotelephone be known within 125 meters, locating a radiotelephone within a one square mile coverage area is not accurate enough.
Conventional electronic location systems are known to track or locate radio transmitters, however, these location systems may not be accurate enough for the needs of the cellular radiotelephone industry. The accuracy of one such conventional system is reduced because the system utilizes cellular radiotelephone signals available on the control channel for location purposes. Control channel signals are of very short duration, sometimes a second or less. Such a system requires more time than this to integrate the signal to determine an accurate location.
Another problem arises when location tracking is desired and the location system being employed utilizes the control channel signals to locate the radiotelephone. The cellular radiotelephone signals used for location purposes in such systems are available only during call setup, making it impossible for repeating location measurements. Additional location measurements may be desired if the caller is moving within the cell or moves into another cell while engaged in a call over the radiotelephone. This condition might arise, for example, if the caller is fleeing from a dangerous situation or if the caller, targeted in a police investigation, is moving about in a vehicle.
The problem of accurately locating a cellular radiotelephone may be exacerbated in the presence of multipath interference. Signal paths to the cellular radiotelephone may include reflections off of buildings in the area. Such reflections can cause the apparent path distance to a fixed cell site to differ from the straight-line distance between the fixed base station and the cellular radiotelephone. This is known as multipath interference. In the presence of multipath interference, some prior art systems may require the presence of location signals to be available even longer to accommodate longer integration times. This may not be possible if radiotelephone signals used for location purposes are of very short duration.
A problem exists with location systems that require excessive equipment or equipment that is incompatible with the existing cellular infrastructure. Excessive equipment in location systems increases costs and implementation time. For example, such systems as GPS and LORAN employ a method for locating transmitters using time difference of arrival (TDOA). The application of GPS receivers or LORAN to cellular radiotelephones includes measuring the time of arrival of the same signal at a multiplicity of locations and comparing the times to determine how long the signal took to reach the receiver. One such system requires the use of a steerable antenna array which is not compatible with existing omnidirectional antenna structures in use with existing cellular base stations. To implement this system additional antenna arrays may be needed which could drive up the cost of implementing such a location system.
Thus, there remains a need for a location system that minimizes changes to existing equipment while providing a method for accurately locating a cellular radiotelephone engaged in a call. Such a system should be able to repeat location measurements as desired throughout the duration of the call even following a cellsite handover of the call.